1. Field of the Invention
This invention relates to a process for removing water from aqueous methanol by azeotropic distillation.
2. Description of the Related Art
Methanol is a widely used industrial chemical used to synthesize many different compounds and industrial products. In several processes utilizing methanol as a synthesizing feed compound, e.g., production of methyl esters, unreacted aqueous methanol is produced as a by-product which must be treated to remove water so that methanol of acceptable purity, i.e., having no more than a certain maximum percentage of water, may be recycled to the process. A common form of purification for removal of water from aqueous methanol is fractional distillation. However, disadvantages of this treatment are that the equipment for carrying out this treatment has a relatively high capital cost and the wastewater produced as a residue cannot have more than a certain proportion of methanol, e.g., about 1000 ppm, in order to meet environmental regulations. Thus, any expedient which can serve to keep the equipment and therefore its cost as small as possible, and the content of methanol in the separated water of the residue as low as possible, is very desirable.
U.S. Pat. No. 3,293,154 issued Dec. 20, 1966 to R. H. Newton, discloses a process in which a methanol-water mixture in vapor form is contacted with a water-insoluble, normally liquid hydrocarbon having a boiling point substantially no higher than the boiling point of methanol, to form an azeotrope with the free water, which azeotrope has a boiling point sufficiently different from that of methanol to accomplish the separation of enough water from the methanol to produce an essentially anhydrous alcohol.
U.S. Pat. No. 3,686,268, issued Aug. 22, 1972 to Jobert et. al., teaches a process for the manufacture of acrylic or methacrylic higher esters by transesterification of a first alcohol with an acrylic or methacrylic ester of a second alcohol having less carbon atoms than the first alcohol in the presence of a titanium phenoxide catalyst, and drawing off the second alcohol by azeotropic distillation with the ester of the second alcohol.
U.S. Pat. No. 3,875,212, issued Apr. 16, 1975 to Ohrui et. al., discloses a process for the synthesis of an acrylic ester such as methyl acrylate by esterifying acrylic acid with methanol or ethanol in the presence of a water-insoluble hydrocarbon solvent while azeotropically boiling water formed in the esterification together with the solvent and distilling the ester simultaneously; and separating the resulting effluent esterification solution into an organic solvent layer and an aqueous layer by decantation.
U.S. Pat. No. 4,076,950, issued Feb. 28, 1978 to Stewart et. al., teaches a continuous process for the preparation of acrylic and methacrylic esters such as methyl acrylate, in which substantially anhydrous product ester is added to the reactor to satisfy the ester/alkanol and ester/water azeotropes thereby removing crude product ester, unreacted alkanol and water of esterification from the esterification reactor while leaving substantially all the high boiling acrylic or methacrylic acid in the reactor.
In accordance with this invention, water is removed from an initial mixture comprising methanol and water by subjecting said mixture to azeotropic fractional distillation in the presence of sufficient added methyl acrylate to form an azeotrope with most of the methanol in said mixture such that a substantial proportion of the methanol in the initial mixture is removed as an overhead azeotrope of methanol and methyl acrylate having an atmospheric boiling point of 62.5xc2x0 C. and containing about 46 weight percent (wt. %) of methyl acrylate and about 54 wt. % of methanol, with a major proportion of the water in the initial methanol-water mixture being removed as a residue of the distillation usually containing a small amount of methanol and no more than a trace of methyl acrylate.
The process of this invention is capable of treating aqueous methanol to obtain a separated water in the residue containing substantially less methanol, and, in some cases an overhead containing significantly less water, than that obtained when no methyl acrylate is added, using similar equipment and energy expenditure, or such process can be used to treat aqueous methanol to obtain a separated water residue containing a similar amount of methanol and an overhead stream containing a comparable percentage of water, than when no methyl acrylate is added, using smaller equipment, and/or a lower expenditure of energy. Alternatively, a combination of these relative advantages may be obtained at various levels of equipment size, energy expenditure and percentage of methanol in the residue water.
It can thus be seen that the process of the invention can yield a significant economic advantage in the lower cost of equipment and/or energy and/or an environmental advantage due to a lower amount of methanol in the residue water if such water is to be discarded.